Control circuits for an electric tractor

ABSTRACT

The main current conduction path of a controlled rectifier and a starting network therefore are connected in parallel across a source of unidirectional potential through a single pole, double throw switch. With the switch in the start position with the pole engaging one contact the starting network is connected in circuit with the source of operating potential and in the other position with the pole engaging the other contact, circuit is completed through the controlled rectifier. The triggering voltage developed by the starting network decays rapidly with time the starting network is out of circuit. Accordingly, after conduction has been established in the controlled rectifier by switching the pole of the switch from the start to the operate position, should conduction be interrupted by a safety switch in the main conduction circuit of the controlled rectifier conduction cannot be reestablished therein by simply closing the safety switch but requires the return of the single pole, double throw switch to the start position to establish triggering potential for the controlled rectifier.

United States Patent Mason et al.

[ 1 Mar. 14, 1972 [54] CONTROL CIRCUITS FOR AN ELECTRIC TRACTOR [72] Inventors: William B. Mason, Schenectady; Douglas R. Houst, Pattersonville, both of NY.

[73] Assignee: General Electric Company [22] Filed: Feb. 12, 1970 [21] App]. No.: 10,863

[52] US. Cl ..318/387, 318/549 [51] Int. Cl. .1102!) 1/00 [58] Field ofSearch .,....318/386, 387,400, 405,406, 318/421, 422, 546, 549

[56] References Cited UNITED STATES PATENTS 2,452,127 10/1948 James ..318/422 2,602,888 7/1952 Audier... .....318/400 3,231,809 1/1966 Greer ..318/400 3,390,318 6/1968 Gilbert ..3l8/422 Primary Examiner--Bemard A. Gilheany Assistant Examiner--Thomas Langer Attorney-Walter C, Bemkopf, Julius .l. Zaskalicky, Frank L. Neuhauser, Oscar B. Waddell and Joseph B. Forman [57] ABSTRACT The main current conduction path of a controlled rectifier and a starting network therefore are connected in parallel across a source of unidirectional potential through a single pole, double throw switch. With the switch in the start position with the pole engaging one contact the starting network is connected in circuit with the source of operating potential and in the other position with the pole engaging the other contact, circuit is completed through the controlled rectifier. The triggering voltage developed by the starting network decays rapidly with time the starting network is out of circuit. Accordingly, after conduction has been established in the controlled rectifier by switching the pole of the switch from the start to the operate position, should conduction be interrupted by a safety switch in the main conduction circuit of the controlled rectifier conduction cannot be reestablished therein by simply closing the safety switch but requires the return of the single pole, double throw switch to the start position to establish triggering potential for the controlled rectifier.

8 Claims, 6 Drawing Figures PATENTEDMAR 14 I972 SHEET 2 OF 4 [r2 van a: was. W/W/bm 5. Mason, Doug/as 2 H0116 b,

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PAIENTEBMAR 14 I872 SHEET t 0F 4 [r7 Vern; ans. W/'///'am B. Mas on, Dog/as R Haas 1,

The/r Azior'ney CONTROL CIRCUITS FOR AN ELECTRIC TRACTOR The present invention relates to control circuits for electri cal apparatus and in particular to circuits for starting, stopping and controlling the operation of an electrically powered tractor and of auxiliary electrical apparatus for use in connection therewith.

An object of the present invention is to provide improvements in the electrical circuits for an electrically powered tractor and for the auxiliary electrical apparatus to be used in connection therewith.

Another object of the present invention is to provide particular kinds of electrical components and locate such components on the tractor and in the electrical circuits thereof to provide maximum operator safety, ease of operation and superior performance.

A further object of the present invention is to provide electrical circuits which enable reliable starting, stopping and operation of the tractor and the accessories thereof.

In carrying out the invention as applied in an illustrative embodiment thereof, there is provided a source of unidirectional voltage having a positive terminal and a negative terminal. Also provided are a silicon controlled rectifier having a cathode, an anode, and control or gate electrode and a starting network for the controlled rectifier having an input circuit and an output circuit for developing in the output circuit thereof a unidirectional control voltage in response to another unidirectional voltage applied to the input circuit thereof.

Means are provided for coupling the control voltage to the control electrode to cathode circuit of the silicon controlled rectifier to render the device conductive when operating voltage is applied to the anode to cathode circuit of the controlled rectifier. The time constant of the starting network is arranged such that the control voltage decays rapidly to a value insufficient to initiate conduction in the control rectifier when the unidirectional voltage applied to the input circuit of the network is removed. A single pole, double throw switch is provided for alternately connecting the main discharge circuit of the control rectifier and the input circuit of the starting network in circuit between the terminals of the source of unidirectional potential. Accordingly, when the starting network is switched out of circuit and the anode to cathode circuit of the controlled rectifier is switched into the circuit across the terminals of the source conduction occurs in the controlled rectifier and concurrently the control voltage in the output circuit of the network decays to a value insufficient to initiate conduction in the control rectifier. A relay is provided in the anode to cathode circuit of the controlled rectifier which actuates a load circuit as long as conduction exists in the anode to cathode circuit. A safety switch is provided in the circuit from the source of voltage to the silicon controlled rectifier for interrupting current flow in the circuit. Accordingly, when the anode to cathode circuit is interrupted, reestablishing the circuit between the terminals and the anode to cathode circuits of the controlled rectifier does not establish current flow in the controlled rectifier.

The features of our invention which we desire to protect herein are pointed out with particularity in the appended claims. The invention itself, however, both as to its organization and method of operation together with further objects and advantages thereof may best be understood by reference to the following description taken in connection with the accompanying drawings wherein:

FIG. 1 is a front perspective view of an electric tractor embodying the invention showing particularly the location of the electrical components of the electrical circuits of the tractor.

FIG. 2 is a rear perspective view of the electrical tractor of FIG. 1.

FIG. 3 is a schematic diagram of the control circuits of the present invention.

FIG. 4 is an exploded perspective view of the speed control assembly for the electric tractor of FIGS. 1, 2 and 3, showing the switches thereon and the manner of their actuation.

FIG. 5 is a plan view of the cam wheel of the speed control assembly of FIG. 4 showing the various positions thereof.

FIG. 6 is a plan view of the electrical switch elements of the speed control assembly of FIG. 4 useful in explaining the operation thereof.

Referring now to FIGS. 1 and 2, there is shown an electrical tractor 20 on which are mounted the various electrical components thereof. The construction of the tractor is described and claimed in a copending patent application Ser. No. 100,866 (RD-3523) assigned to the same assignee of the present invention. The source of unidirectional operating voltage 21 is a group of storage batteries located in a forward compartment on the main chassis member of the tractor and also in a rear battery compartment underneath the seat of the tractor. An electric motor 22 for driving the rear wheel assembly of the tractor is secured to the underside of the chassis member and is coupled mechanically to the rear wheel assembly. A speed control assembly 23 is provided on a centrally located upright bulkhead member 24 for starting, stopping and controlling the speed and direction of the electrical motor 22. As shown in greater detail in FIG. 4 the speed control assembly comprises a lever 25 which is movable in a slot 26. The slot has a generally horizontal portion 27, a rearward extending portion 28 and a forward extending portion 29. The control lever 25 has 12 positions shown so marked adjacent the slot 26. Positions 4 and 5 represent neutral positions, positions 6 through 12 represent various forward speed positions, positions 3 to 1 represent various reverse speed positions. Associated with the control lever 25 are eight switches, seven of which are shown in schematic form in FIG. 6, and the eighth of which is shown in dotted outline in FIG. 4. The switch elements of FIG. 6 and FIG. 4 starting at the top of the assembly along with their designation and an indication of kind are respectively the following:

The eighth switch which functions as a speed reversing switch is a normally opened switch and is designated T-REV.

Also shown on the member 24 is a power takeoff switch designated PTO consisting of a single pole, double throw switch for operation of auxiliary apparatus such as lifts, mowers, and the like. A disconnect switch designated LD for rapid disconnect of the circuits of the tractor from the source of voltage is located on the member 24. The seat support member 30 is hinged in the rear to support member 31 and is spring biased upward so that when an operator is not sitting on the seat 32 secured to member 30, the seat is inclined upward. A switch 33 is mounted to the front side of seat support member 31 and has a plunger 34 engaging the member 30 so that when the seat is up a pair of contacts are opened and when the seat is down a pair of contacts are closed. Another switch 35 is located adjacent the brake pedal and assembly 36 has a normally closed pair of contacts which are opened by actuation of the brake pedal thereof. A switch 37 is provided for operation of winch 38 for lift assembly 38 of the tractor. Also a key switch 39 for connecting power to the electrical circuits is provided. An ammeter 40 and a volt meter 41 are also located on the member 24. An outlet 42 is located on the battery compartment of the tractor for insertion of cables from an auxiliary apparatus to be used in connection with the tractor.

The electrical components, the locations of which are described above in connection with FIGS. 1 and 2 are shown in the overall electrical schematic of the electrical tractor of FIG. 3 to which reference is now made and are designated by the same reference symbols as used in connection with FIGS. 1 and 2. Also shown in the circuit of FIG. 3 are a plurality of relays and their contacts. Relay L includes a solenoid L and a normally open pair of contacts L-C. Relay 1A includes a sole noid 1A and a pair of normally open contacts lA-C. Relay 2A includes a solenoid 2A and a pair of normally open contacts 2 A-C. Relay FW includes a solenoid FW and a pair of normally closed contacts FW-C. Relay REV includes a solenoid REV and two pairs of normally closed contacts REV-C1 and REV- C2 and two pairs of normally open contacts REV-(E3 and REV-C4.

The positive terminal 50 of the source 21 is connected through the line disconnect switch LD, a thermal circuit breaker CB1, the normally open contacts L-C, and through resistors R1 and R2 connected in series to one terminal of the armature 51 of the shunt motor 22. The other terminal 52 of the armature is connected to the negative terminal 53 of the source 50. An ammeter 40 is provided in the latter circuit. Normally open contacts 1 A-C are connected in shunt with resistance R1 and normally open contacts 2 A-C are connected in shunt with resistance R2. The field circuit of the shunt motor 22 includes a field winding 54 and resistors R4, R5, R6, R7 connected in series. In the forward direction of rotation of the motor, one terminal 55 of the field winding is connected through the normally closed contacts REV-Cl to the contacts L-C remote from the positive terminal 50 The other terminal 56 of the field winding is connected through the other pair of normally closed contacts REV-C2 to the remote end of field weakening resistor R4. The other end of the series connected string of the field weakening resistors R4, R5, R6 and R7, namely, the remote end of resistor R7 is connected to the negative terminal 53 of the source. Normally open contacts REV-C3 are connected between terminal 55 of the field winding and the aforementioned remote end of resistance R4 and the other normally open REV-C4 of contacts is connected between the terminal 56 and the aforementioned remote contact of contacts L-C. Switches T-FWl, T-FW2, T-FW3, T- FW4 are connected in shunt, respectively, with resistances R4, R5, R6 and R7.

Relay actuation circuits are connected between a point 60 between the junction of the thermal circuit breaker CB1 and the line contacts L-C and the negative terminal 53 of the source. Connected in series between the point 60 and the pole 61a of the single pole, double throw switch T-Start are a two terminal key switch 39, a two terminal seat switch 33, and a two terminal brake switch 35. One contact 61 of the single pole, double throw switch T-Start is connected to the anode 62 of the silicon controlled rectifier 63, the cathode 64 of which is connected to one terminal of the solenoid L. The other terminal of the solenoid L is connected to the negative terminal 53. The other contact 65 of the single pole, double throw switch is connected to the anode of unilaterally conducting device 66, the cathode of which is connected to one terminal of starting network 67. The other terminal of the starting network is connected to the cathode 64 of the control rectifier. The starting network consists of a capacitor 68 with series connected divider resistances 69 and 70 in parallel therewith. The junction point 71 between the resistances 69 and 70 is connected to the gate or control electrode 72 of the controlled rectifier. The time constant of the starting network is selected to be long enough such that sufficient voltage appears across resistance 70 to trigger the controlled rectifier 63 after the time interval of switching the pole 61a from contact 65 to contact 61. However, the time constant is selected not to be so long as would negate the safety function of the circuit as will be explained below. For a switch having a switching time of 50 milliseconds from one contact to the other, retention of adequate triggering voltage across resistance 70 for 100 milliseconds would be satisfactory and provide desired safety functions.

Each of the circuits 80, 81 and 82 for controlling current flow through solenoids 1A, solenoid 2A and solenoid FW, respectively, are identical and each includes a positive and a negative terminal. The circuit for controlling the actuation of solenoid 1A comprises a silicon controlled rectifier 85 having an anode 86 connected to the positive terminal 83 and a cathode 87 connected to the negative terminal 84. Series resistance 88 and capacitance 89 are connected in the order named between the positive terminal 83 and the negative terminal 84. The positive base 90 of a unijunction transistor 91 is connected through a load resistance 92 to the positive terminal 83 and the negative base 93 ofthe unijunction transistor is connected through the resistor 94 to the negative terminal 84. The emitter or control electrode 95 of the unijunction transistor is connected to the junction of resistance 88 and capacitance 89. In operation, upon the application of operating potential between the positive terminal 83 and negative terminal 84, the capacitor 89 charges, as determined by the time constant of the resistance 88 and capacitance 89 until a voltage is reached which triggers the unijunction transistor 91 to pass current between the base electrodes thereof thereby establishing a triggering potential across the resistance 94 which triggers the silicon controlled rectifier 85 conductive. Accordingly, the circuit provides a conduction path after the elapse of a predetermined time from the application of potential between the positive and negative terminals thereof. The positive terminal 83 of the network is connected to one terminal of the switch T-lA the other terminal which is connected to the cathode 64 of silicon controlled rectifier 63.

The negative terminal 84 of the circuit is connected to one of terminal of the switch T-2A, the other terminal which is connected to the positive terminal 95 of the circuit 81. The negative terminal 96 of the circuit is connected to one terminal of the solenoid 2A, the other terminal of which is connected to terminal 53. Terminal 96 is also connected to the positive terminal 97 of the circuit 82, The negative terminal 98 of circuit 82 is connected to one terminal of the solenoid FW, the negative terminal of which is connected to the negative terminal 53.

The circuits 80, 81 and 82 are sequentially firing circuits for sequentially energizing the solenoids of relay 1A, 2A and FW according to a predetermined time period. Accordingly, when switches T-lA and T-2A are closed, initially silicon controlled rectifier 85 is fired after a predetermined time interval determined by the time constant of the circuit 80. After another predetermined time interval the silicon controlled rectifier of circuit 81 is fired and actuates solenoid 2A and after a third predetermined time determined by the time constant of circuit 82 the silicon controlled rectifier thereof is fired and actuates solenoid FW.

The pole of switch T-Start is connected to one contact of switch T-REV, the other contact of which is connected to the positive end of solenoid REV. The negative end of solenoid REV is connected to the negative terminal 53 of the source. A series circuit of resistance 100 and capacitance 101 is connected in shunt with the solenoid REV to provide a time delay in the discharge of the solenoid.

ln operation of the circuits described above, the line disconnect LD is put in place, the thermal circuit breaker CB1 being associated with the motor 22 is closed, the key switch 39 is closed, the operator in sitting on the seat of the tractor actuates the seat switch 33 and the brake lever is released to close brake switch 35. The T-Start switch is placed in the start position with the pole thereof engaging contact 65 to charge the triggering or starting network 67 and then it is placed in the forward operate position 6 with the pole thereof engaging contact 61 to provide operating potential to the silicon controlled rectifier 63 thereby causing current to flow therethrough to energize the solenoid L. The closing of the contacts L-C provides voltage across the armature circuit of the motor 22 through resistances R1 and R2. The lever 25 is moved into position 7 causing contacts lAC to close after a short time delay determined by the gate triggering circuit of the controlled rectifier 85. The closing of contacts lAC causes resistance Rl to be shorted out of circuit thereby increasing the speed and torque of the motor 22. Upon moving the lever 25 to position 8 the switch T-2A is closed providing operating voltage across the circuit 80 and after a predetermined time delay in the unijunction triggering circuit the silicon controlled rectifier of the circuit is energized thereby closing contacts 2AC and shorting resistance R2 and hence further increasing the speed and torque of the motor. After a predetermined interval of time of energization of the relay 2A, the relay FW is energized through the circuit 82 opening up the normally closed contacts FW-C connected across the series string of field resistances R4, R5, R6 and R7. it should be noted with the closing of line contacts L-C the field circuit of the motor 22 is completed through the normally closed contacts REV-C1 and REV-C2 of relay REV. As the lever 25 is moved into positions 9, 10, 11 and 12 the normally closed contacts of the switches T-FWl, T-FW2, T-FW3 and T-FW4 are, respectively opened thereby respectively inserting resistances R4, R5, R6 and R7 in series circuit with the field winding 54 reducing the current flow therethrough thereof and increasing the speed of the motor. To reverse the direction of rotation of the motor 22, the lever 25 is brought back to position 4, moved generally horizontally in the guide slot 26 and moved rearward in the portion 28 thereof. When the lever 25 is moved to position 5 and position 4, the switches T-FW4, T-FW3, T-FWZ, T-FWl, T-2A, TlA and T- Start are returned to their normal positions and line contactors L-C are opened. When the lever 25 is moved from position 4 to slot 29 to position 5 in slot portion 28 it traverses slot portion 27 and closes switch T-REV (See FIG. 4). Closing of switch T-REV energizes the relay REV which reverses the polarity of the terminals of the field winding 54. When the control lever is moved into position 3, the T-Start operate switch is moved from the start to the operate position and causes energization of the silicon controlled rectifier 63 as in the case of forward direction operation. Moving the control lever to position 2 closes the contacts of switch T-lA and similarly moving the control lever to position 1 closes the contacts of switch T-2A to sequentially short out resistances R1 and R2 in series with the armature circuit of the motor 22. When the lever 25 is moved back to position 4 and position 5, the switches T-ZA, T-lA, and T-Start are returned to their normal positions and line contactors L-C are opened. When the lever 25 is moved to traverse slot portion 27 from slot portion 28 to slot portion 29 the switch T-REV is opened. The location ofswitch T-REV so as to be actuated and deactuated during the traverse of lever 25 in slot 28 is to assure positive operation of the field reversing contacts and operation of such contacts only after the line contacts L-C have been opened and energy of the motor system has been allowed to decay sufficiently. Separation of the forward speed and reverse speed positions by the generally horizontal slot portion 27 also provides an enforced time delay in going from one speed direction to the opposite speed direction. A delay is provided in the release of normally opened contacts REV-C3 and REV-C4 by capacitor 101 and resistance 100 connected in series across the solenoid of relay REV. Such delay maintains the field winding 54 in circuit until the energy of the motor system has been dissipated and avoids undesired arcing of the contacts. The organization of the switch elements of assembly 23 including the lever 25 to effect the switching action described in the operation of the circuit for operating the armature motor at various forward speeds and various reverse speeds will be explained below in connection with FIGS. 4, 5 and 6.

A particular feature of the circuit described is that should the operator of the tractor apply the brake during any one of the forward or reverse speed settings of the lever 25, the current flow through the silicon controlled rectifier 63 is interrupted as the brake switch 35 is opened. Now should the operator release the brake so as to close the switch 35 the tractor will not start as only cathode to anode voltage is being applied without appropriate triggering of voltage being applied to the gate of the controlled rectifier. The triggering voltage of network 67 has decayed below a triggering level shortly after pole 61a engages contact 61 as explained above. Accordingly, to start the tractor it is necessary to return the control lever 25 to one of the neutral positions 4 and 5 to apply voltage to the input circuit of the starting network 67 to establish a triggering voltage to trigger the control rectifier. The seat switch 33 is also located in series with the current flow paths from a source 21 of unidirectional voltage to the control rectifier 63 so that should the operator rise from the seat the circuit is broken and sitting back down again does not reestablish current to the armature of the motor provided the operator does not sit back down practically instantaneously while sufficient triggering voltage is available from the triggering network making the time constant of the triggering network short as explained above as a practical matter eliminates the latter possibility. As in the case of the brake switch operation it is necessary to return the lever 25 to a neutral position and initiate the start-operate sequence as explained above.

Another feature of the present invention is the sequentially operated relay circuits 80, 81 and 82 for sequentially applying energization to the speed control relays 1A, 2A and F W of the motor to increase the speed thereof. Should the operator move lever 25 rapidly through positions 6, 7 and 8, the tractor would not accelerate in accordance with such movement. lts acceleration from one position to the next would be determined by the time constants of the unijunction triggering circuits thereof. Of course, the time constants of the circuits 80, 81 and 82 can be made different and are designed so as to avoid too rapid a switching from one set of speed versus torque characteristics to the next set of speed vs. torque characteristics in order to provide smooth acceleration. A capacitor is connected in shunt with the field winding 54 to suppress transients produced in the field circuit which could damage the solid state active devices ofthe circuit such as the controlled rectifiers and unijunction transistors. A damping circuit consisting of a resistance 106 and a reversely poled diode 107 is connected in shunt with each of the relays L-lA, 2A, FW and REV to permit discharge of the inductance of the solenoids upon breaking of the current flow through the solenoids.

A power takeoff device 110, such as the motor of an electric mower, is connected in series with a thermal circuit breaker CB2. The terminals of such series combination are then connected to a pair of outlet terminals on socket 42. The power takeoff PTO has a solenoid PTO and a set of normally open contacts PTO-C1 and a set normally closed contacts PTO-C2. The contacts PTO-C1 and PTO-C2 are connected in series across the source 21 of operating voltage. One outlet terminal 111 is connected to one terminal of the normally closed contact PTO-C2 and the other outlet terminal 112 is connected to the other normally closed contact PTO-C2. A power takeoff switch PTO-S is provided. The switch PTO-S is a single pole, double throw switch having a pole 115 and a pair of terminals 116 and 117. A circuit similar to the circuit used for actuation of the solenoid relay L is used for the actuation of the solenoid PTO of the relay PTO. The circuit comprises a silicon controlled rectifier 120 having an anode 121 connected to terminal 117 of switch PTO-S, the cathode 122 of which is connected to one terminal of the solenoid PTO. The other terminal of the solenoid PTO is connected to the negative terminal 53 of the source. A starting circuit 123 for the control rectifier comprises a capacitance 124 and a pair of resistances 125 and 126 connected in series across the capacitor 124. The junction point of resistance 125 and 126 is connected to the gating or control electrode 127 of the silicon controlled rectifier 120. One terminal of the capacitor 124 is connected to the cathode of unilaterally conducting device 128, the anode of which is connected to terminal 116 of PTO-S switch. The other terminal of the capacitor 124 is connected to the cathode of the silicon controlled rectifier. The operation of the circuit described is identical to the operation of the control circuit for actuating of the relay L. It is initially necessary to switch the pole 115 ofthe PTO-S switch to the terminal 116 to establish a starting potential on the gating electrode 127 of the controlled rectifier and then switch the pole 115 to contact 117 to establish operating potential to the silicon controlled rectifier 20, thereby initiating conduction therein and conduction in the PTO relay. The normally open contacts PTO-C1 are closed and the normally closed contacts PTO-C2 are opened. thereby applying power to the motor 110. Should power to the silicon controlled rectifier 120 be interrupted by the operator arising from the seat. the PTO relay circuit is opened up and hence the normally opened contacts PTO-C1 thereof becomes normally open and the normally closed contacts PTO-C2 become closed providing a dynamic braking circuit for the motor 110 thereof. Closing the circuit through the silicon controlled rectifier 120 by closing the seat switch does not establish conduction in the controlled rectifier and does not establish operation of the power takeoff device. It is necessary to switch the pole 115 of the PTO-S switch to terminal 116 before conduction can be established to the power takeoff motor 110.

Referring now to FIG. 4, there is shown an exploded view of the speed control assembly 23 described in FIGS. 1. 2 and 3. The speed control assembly 23 has a frame support 130 member including a top generally planar portion 131 and a side generally planar portion 132. The face cam wheel 133 of plastic material is rotatably supported by brackets 134 and 135, fastened to the side portion 132 to rotate about an axis perpendicular to the side portion 132. On an outwardly extending face 136 of the wheel 133 are included a plurality of narrow elevated strips extending for various circumferential lengths and located in various radial positions thereon.

Reference is now made particularly to FIG. which shows a plan view of the outward extending face 136 of wheel 133. The center line of cams 140 and 141 are located on the same radius, that is, an outermost radius. Cams 142 and 143 are located at the same radius smaller than the radius of cams 140 and 141. Similarly, cams 144 and 145 are located at the same radius smaller than the radius of cams 142 and 143. Cams 146 tached to the face wheel on the side opposite to which the cams are located and is pivotally secured to one end of the shaft 153. The shaft 153 is pivotally mounted between side portion 132 and bracket 135. The axis of rotation of the shaft 153 is parallel to the axis of rotation of the cam wheel 133. To the end of the shaft opposite to which the sector gear 152 is secured, is attached the lever 25. The lever 25 is pivotally mounted on shaft 153 by means ofa pin 155 extending along a diameter of the shaft 153 so as to permit forward and reverse movement of the lever to produce a rotation of the shaft 153 and hence a rotation of the wheel 133 and a sideways movement of the lever 25 about the axis of the pin which does not produce any axial rotation of the wheel 133. The lever 25 extends through the slot 26 in the top assembly portion 130 of the frame. The slot 26 has a forward extending elongated portion 29 and a rearward extending portion 26 displaced sideways therefrom. The upper forward portion of slot 26 and a rear portion of slot 29 are connected by a generally horizontal or sideways extending portion 27 inclined slightly in the forward direction. The numbers adjacent the marker lines touching on the slot 26 indicate positions of the lever 25 corresponding to positions of diametrical lines so numbered on the face wheel of FIG. 5 in registry with the axis of the switch assembly. In the drawing of FIG. 4 the lever position is shown in position 4. On FIG. 5 the line 4 intersects no cams so that no switches are actuated. When lever 25 is in position 1, cams 141, 143 and 142 actuate switches T-Start, T-IA and T-ZA. Similarly, for the other positions various switches are actuated. Such actuation is indicated on the following diagram which shows the switches of the assembly 23 as well as FW relay and T-REV in columns and the lever positions 1 through 12 in rows. An X at the intersection ofa column and a row indicates actuation of the component for the switch position.

Switch and 147 are located on the same radius closest to the center of the rotational axis of the wheel 133. The radii and circumferential extent of the cams 140-147 are arranged so as to engage particular ones and over a particular angle of displacement of the wheel 133 of the buttons of switches T-2A. T- FW4, T-FW3, T-FW2, T-FWI, T-lA, and T-Start. The cams function to actuate the buttons of the aforementioned switches which are mounted in a line along a diameter of the face of the wheel 133. The switches are mounted to the bracket or cross member 134 which in turn is secured to the side portion 132. The particular location of the switches is shown in FIG. 4 and also shown in FIG. 6. Starting from the lower left hand corner of FIG. 4 the switches are, respectively, the following T-Start switch, T-lA, T-FWl, T-FW2, T-FW3, T-FW4, and T-2A.

FIG. 6 shows a schematic of the aforementioned switches and also shows the order of their arrangement. The cam wheel 133 may be oriented in any one of twelve positions with respect to the axis of the switch assembly and are so designated numerically on FIG. 5. Each of the positions are equally spaced from adjacent positions and are locked in place by a detent in the periphery of the wheel 133 which engages a spring loaded ball 150 secured to side portion 132. The face cam wheel 133 is manually positioned by means of a leverage assembly 151 comprising a sector gear 152, a shaft 153 and lever 25. The sector gear 152 engages a spur element 154 at- Additionally shown in the diagram are the action switch T- REV for positions 1. 2 and 3 and for neutral positions located in slot portion 28. Also shown is the actuation of the field weakening relay FW for the indicated positions.

While the invention has been described in a specific embodiment, it will be appreciated that many modifications may be made by those skilled in the art and we intend by the appended claims to cover all such modifications and changes as fall within the true spirit and scope of the invention.

What we claim as new and desire to secure by Letters Patent of the United States is:

1. In an electrical network wherein a source of unidirectional voltage is coupled to a load circuit, such as a DC motor, and wherein actuation of current interrupting means, such as contactors. from a first to a second state interrupts application of unidirectional voltage from said source to said load circuit, a reset control arrangement requiring manual resetting to reapply the unidirectional voltage from said source to said load circuit, comprising:

a. said source of unidirectional voltage having a positive terminal and a negative terminal;

b. a unidirectional conduction control device having a cathode, an anode, and a control electrode;

c. a starting network comprising an input adapted to be coupled to said source of unidirectional voltage, an output, and means for producing in said output a unidirectional control voltage decaying rapidly subsequent to removal of unidirectional voltage from the input circuit of said network;

d. means coupling said output to the control electrode to cathode circuit of said control device to render said device conductive when operating voltage is applied to said anode to cathode circuit thereof and to preclude the initiation of conduction in said device subsequent to decay of said control voltage;

e. switching means for alternately connecting said anode to cathode circuit and said input of said starting network in circuit between said terminals of said source, whereby when said network is switched out of circuit and said anode to cathode circuit is switched into circuit across said terminals conduction occurs in said anode to cathode circuit and the control voltage in said output circuit of said network decays to a value insufficient to initiate conduction in said control device;

f. said load circuit,

g. means responsive to conduction in said anode to cathode circuit to actuate and maintain conduction in said load circuit as long as conduction exists in said anode to cathode circuit, and

h. means for interrupting current flow in said anode to cathode circuit, whereby said load circuit is opened and closing the circuit between said terminals and said anode to cathode circuit path does not reestablish current flow through said control device.

2. The combination of claim 1 in which said switching means is a single pole double throw switch having a pole and a pair of contacts, one of said contacts connected to one of the main current conduction electrodes of said control device, the other of said contacts connected to one of the terminals of said starting network, the other of said main current conduction electrodes of said control device and the other of said terminals of said network connected in circuit to one of said terminals of said source and said pole connected in circuit to the other of said terminals of said source.

3. The combination of claim 1 in which said load circuit includes an electric motor.

4. The combination of claim 1 in which said unidirectional conduction control device is a silicon controlled rectifier having an anode, a cathode and a control electrode.

5, The combination of claim 1 in which said starting network includes resistive and capacitive elements.

6. In an electrically driven tractor the reset control arrangement of claim 1 wherein said load circuit means comprises the drive motor circuit of said tractor.

7. The combination of claim 6 in which said current flow interrupting means includes a switch responsive to the actuation of the tractor braking mechanism to open said switch.

8. The combination of claim 6 in which said current flow interrupting means includes a switch responsive to the presence of an operator on the operator seat thereof to close said switch and responsive to the absence of an operator from the operator seat thereof to open said switch. 

1. In an electrical network wherein a source of unidirectional voltage is coupled to a load circuit, such as a DC motor, and wherein actuation of current interrupting means, such as contactors, from a first to a second state interrupts application of unidirectional voltage from said source to said load circuit, a reset control arrangement requiring manual resetting to reapply the unidirectional voltage from said source to said load circuit, comprising: a. said source of unidirectional voltage having a positive terminal and a negative terminal; b. a unidirectional conduction control device having a cathode, an anode, and a control electrode; c. a starting network comprising an input adapted to be coupled to said source of unidirectional voltage, an output, and means for producing in said output a unidirectional control voltage decaying rapidly subsequent to removal of unidirectional voltage from the input circuit of said network; d. means coupling said output to the control electrode to cathode circuit of said control device to render said device conductive when operating voltage is applied to said anode to cathode circuit thereof and to preclude the initiation of conduction in said device subsequent to decay of said control voltage; e. switching means for alternately connecting said anode to cathode circuit and said input of said starting network in circuit between said terminals of said source, whereby when said network is switched out of circuit and said anode to cathode circuit is switched into circuit across said terminals conduction occurs in said anode to cathode circuit and the control voltage in said output circuit of said network decays to a value insufficient to initiate conduction in said control device; f. said load circuit, g. means responsive to conduction in said anode to cathode circuit to Actuate and maintain conduction in said load circuit as long as conduction exists in said anode to cathode circuit, and h. means for interrupting current flow in said anode to cathode circuit, whereby said load circuit is opened and closing the circuit between said terminals and said anode to cathode circuit path does not reestablish current flow through said control device.
 2. The combination of claim 1 in which said switching means is a single pole double throw switch having a pole and a pair of contacts, one of said contacts connected to one of the main current conduction electrodes of said control device, the other of said contacts connected to one of the terminals of said starting network, the other of said main current conduction electrodes of said control device and the other of said terminals of said network connected in circuit to one of said terminals of said source and said pole connected in circuit to the other of said terminals of said source.
 3. The combination of claim 1 in which said load circuit includes an electric motor.
 4. The combination of claim 1 in which said unidirectional conduction control device is a silicon controlled rectifier having an anode, a cathode and a control electrode.
 5. The combination of claim 1 in which said starting network includes resistive and capacitive elements.
 6. In an electrically driven tractor the reset control arrangement of claim 1 wherein said load circuit means comprises the drive motor circuit of said tractor.
 7. The combination of claim 6 in which said current flow interrupting means includes a switch responsive to the actuation of the tractor braking mechanism to open said switch.
 8. The combination of claim 6 in which said current flow interrupting means includes a switch responsive to the presence of an operator on the operator seat thereof to close said switch and responsive to the absence of an operator from the operator seat thereof to open said switch. 